Quick drop valves are commonly used in hydraulic control systems for bulldozer blades, or the like, in which the blade is allowed to freefall to ground level under the force of gravity. Some of the fluid expelled from the double acting hydraulic actuators which control blade elevation is diverted by the quick drop valves to the expanding ends of actuators to supplement the pump flow thereto. Without any type of quick drop valve, the expanding ends of the actuators cavitate quite badly. Since the cavitated ends of the actuators have to be filled with fluid from the pump after the blade comes to rest on the ground, a considerable time lag occurs before sufficient downward force can be applied to the blade for penetrating the ground. The use of quick drop valves minimizes the cavitation and, thus, reduces the time lag.
The known quick drop valves are moved to and retained in the quick drop position by differential pressure generated by the expelled fluid passing through a triggering orifice once the flow rate of the expelled fluid exceeds a predetermined rate. The size of the orifice usually dictates how quickly sufficient differential pressure is generated to move the valve to the quick drop position and how much of the expelled fluid can be diverted to the expanding ends of the actuators. One of the problems encountered with the use of the triggering orifice is that at least some of the expelled fluid must pass through the trigging orifice in order to maintain a pressure differential sufficient to hold the quick drop valve in the quick drop position. The fluid passing through the orifice is directed back to the tank and, thus, cannot be used to help fill the expanding end of the actuator such that efficiency of the quick drop valve is compromised.
Another problem encountered with the known quick drop valves is that in one mode of operation the blade is allowed to freefall from the raised position and then suddenly stopped before the blade reaches the ground in order to shake loose any material adhering to the blade. With the known quick drop valves, however, the main control valves must be moved essentially back to the neutral position before the quick drop valve can move back to its non-quick drop position.
Thus, it would be desirable to have a quick drop valve circuit constructed so that all of the fluid expelled from the actuators is diverted back to the expanding ends of the actuators. It would also be desirable to have a quick drop valve circuit in which the quick drop valve can be quickly returned to its non-quick drop position before the main control valve reaches the neutral position.
The present invention is directed to overcoming one or more of the problems as set forth above.